The present invention relates to thermobondable polyolefin-containing synthetic fibres or filaments, in particular cardable staple fibres, treated with hydrophobic spin finishes comprising an antistatic agent and a hydrophobic agent, a method for producing the fibres or filaments, and nonwoven products prepared from the fibres or filaments.
The fibres, which have the advantage of being able to be carded at high speeds, are particularly suitable for use in the preparation of thermally bonded hydrophobic nonwoven fabrics in which a dry, water repellent surface which can function as a liquid barrier is desired, e.g. for disposable diapers and feminine hygienic products. The fibres are also suitable for the preparation of thermally bonded nonwoven fabrics for medical use in which a dry, water repellent surface is desired in order to reduce bacterial penetration, for example medical gowns and drapes.
Hygienic products such as disposable diapers, sanitary napkins and adult incontinence pads generally have barriers through which fluids absorbed by the absorbent core are not able to penetrate, e.g. in the form of side guards or other structural elements or as back sheet material opposite to the skin. Such barriers may comprise a nonwoven material prepared from hydrophobic staple fibres or a spunbonded material prepared directly from a hydrophobic polymer. However, spunbonded materials are relatively flat and film-like, and do not have the soft, uniform, textile-like comfort that one finds in carded nonwovens. Spunbonded fabrics are therefore not the optimal choice for liquid barriers designed to be in contact with the skin of the user. Also, spunbonded nonwovens have a non-uniform distribution of fibres, which results in weak areas (holes) that limit the liquid barrier properties of the fabrics, so that web uniformity becomes the limiting factor for the hydrophobic characteristics. As for nonwovens prepared from staple fibres, these tend not to be sufficiently hydrophobic for such liquid barriers, due to the fact that during the spinning process, the fibres are treated with a xe2x80x9cspin finishxe2x80x9d which facilitates the spinning and subsequent carding process by lubricating the fibres and making them antistatic. However, as a result of the spin finish treatment, in particular the use of an antistatic agent, which by nature is more or less hydrophilic, polyolefin fibres, although inherently hydrophobic, become somewhat hydrophilic, which in the present context is undesirable. On the other hand, fibres with the desired degree of hydrophobicity have generally had less than optimal antistatic properties.
Nonetheless, a number of polyolefin-containing hydrophobic synthetic fibres are known, for example hydrophobic textile fibres with dirt and stain resistant properties. However, such fibres generally contain cationic antistatic agents that are undesirable or unsuitable for personal hygiene and medical products for toxicological reasons, since they often exhibit skin irritating properties due to their low pH. Also, some components may during use release di- or tri-ethanolamine, which is suspected of causing allergic reactions. It is also known to treat polyolefin fibres with a finish containing a polysiloxane as the hydrophobic agent, but nonwovens obtained from such fibres often suffer from poor tear strength. It has previously proved difficult to produce fibres for hygienic or medical use with satisfactory hydrophobic properties while maintaining good cardability properties and high nonwoven tenacities. This is particularly important for the many applications in which it is desired that hydrophobic fibres may be carded using high carding speeds.
U.S. Pat. No. 4,938,832 discloses a method for preparing hydrophobic polyolefin-containing fibres or filaments in which spun fibres or filaments are treated with a first modifier composition containing 70-100% by weight of at least one neutralised phosphoric acid ester containing a lower alkyl group and up to 30% by weight of at least one polysiloxane with hydrophobic end groups, followed by treatment with a second modifier composition containing 70-100% by weight of the polysiloxane and up to 30% by weight of the neutralised phosphoric acid ester.
EP 0 486 158 A2 discloses a somewhat similar method for preparing hydrophobic polyolefin-containing fibres or filaments, in which spun fibres or filaments are treated with a first modifier composition containing 0-40% by weight of at least one neutralised phosphoric acid ester containing a lower alkyl group and 60-100% by weight of at least one polysiloxane with hydrophobic end groups, followed by treatment with a second modifier composition containing 50-100% by weight of the neutralised phosphoric acid ester and 0-50% by weight of the polysiloxane.
EP 0 516 412 A2 discloses a method for treating the surface of polyolefin-containing fibres to improve their lubricity and antistatic properties by applying a liquid lubricating finish containing an alkylated polyol or a water-soluble ester or polyester obtained by reacting the polyol with a fatty acid having up to 6 carbon atoms. A polysiloxane and a neutralised phosphoric acid ester may also optionally be applied to the fibres.
EP 0 557 024 A1 discloses polyolefin fibres treated with an antistatic agent which is a neutralised phosphate salt, and optionally with a lubricant selected from mineral oils, paraffinic waxes, polyglycols and silicones, the fibres having an hydrostatic head value of at least 102 mm.
Japanese patent publication No. 4-24463/1992 (application No. 86/84081) discloses polyester fibres coated with a spinning oil consisting of 40-85% by weight of at least one neutral oil with a melting point of 30-150xc2x0 C., 5-30% by weight of a cationic surfactant, and the balance of an emulsifier. As discussed above, the use of cationic surfactants is, however, undesirable in products designed for personal hygiene or medical use.
WO 94/20664 describes a method for producing cardable, hydrophobic polyolefin-containing staple fibres using two spin finishes, in which the second spin finish is a dispersion comprising an antistatic agent, preferably an anionic or non-ionic antistatic agent, and, as a hydrophobic agent, a natural or synthetic hydrocarbon wax or wax mixture, and optionally a silicone compound.
WO 95/19465 describes a method for producing cardable, hydrophobic polyolefin-containing staple fibres by applying, after spinning, a first spin finish composition containing at least one cationic antistatic agent and, after stretching, a second spin finish composition containing at least one hydrophobic lubricant chosen from a fatty acid amide condensation product and a hydrocarbon wax.
The fibres described in the above-mentioned publications all have various combinations and degrees of hydrophobic and antistatic properties. However, there is still a need for polyolefin fibres with optimum hydrophobic and antistatic properties for the preparation, in particular by means of high-speed carding, of nonwovens with optimum strength and hydrophobic characteristics.
An object of the present invention is therefore to provide hydrophobic thermobondable synthetic fibres, in particular for hygienic applications, with both optimum hydrophobic and antistatic properties, and thus with improved carding properties suitable for preparation of nonwovens showing superior strength and softness. A further object of the present invention is to improve the application and distribution of spin finish on the fibres by carefully controlling the surface properties of the as-spun filaments, thus improving fibre uniformity, allowing increased carding speed and improved web uniformity in the carding process, which in turn results in nonwovens with improved hydrophobic properties.
In one aspect, the present invention relates to a method for producing hydrophobic polyolefin-containing fibres or filaments, in particular cardable staple fibres, the method comprising the following steps:
a. melt spinning a polyolefin-containing material to produce spun filaments,
b. applying to the spun filaments a first spin finish with an active ingredient content comprising 20-100% by weight of at least one water-insoluble ester of a mono-, di- tri- or tetrahydric alcohol with a molecular weight not exceeding 500 and a branched or straight chain fatty acid with between 12 and 30 carbon atoms,
c. stretching the filaments,
d. applying to the stretched filaments a second spin finish with an active ingredient content comprising 20-100% by weight of at least one water-insoluble ester of a mono-, di- tri- or tetrahydric alcohol with a molecular weight not exceeding 500 and a branched or straight chain fatty acid with between 12 and 30 carbon atoms,
e. optionally, crimping the filaments,
f. applying, during the spinning stage, the stretching stage or after crimping, an antistatic agent,
g. drying the filaments, and
h. for the production of fibres, cutting the filaments to obtain staple fibres.
Further aspects of the invention relate to texturized, cardable, polyolefin-containing fibres produced by the above method, as well as hydrophobic nonwoven materials containing such fibres.
The fibres of the present invention have been found to have excellent hydrophobic properties as well as excellent antistatic properties and can therefore be carded at high carding speeds comparable to carding speeds typically used for hydrophilic staple fibres. It has furthermore been found that webs prepared from the fibres have a uniform distribution of the fibres in both the machine direction and the transverse direction, and that when these webs are thermobonded by calender bonding, nonwovens with improved strength and excellent hydrophobicity are obtained.